Serious drought may strike western US

The western US may be heading towards a return to the dustbowl landscape that devastated the prairies of the 1930s, climatologists warn.

The horror of that period in the US was vividly described in John Steinbeck's novel The Grapes of Wrath: thousands of people migrated from the parched American prairie - a dustbowl - and travelled west towards the "promised land". But the once-greener pastures are heading towards a drought more severe than the one that created the dustbowl of the 1930s.

For the past seven years, states in the western US have been drying up. Rising temperatures, declines in annual precipitation, and an increasing population have combined to leave major water sources running perilously low.

Now, a new analysis by Richard Seager at Columbia University in New York, US, and colleagues suggests the region is in the early stages of a profound shift in climate that may last for decades.

The team employed 19 major climate models used by the Intergovernmental Panel on Climate Change. The models show that sea surface temperatures up to 40 degrees north and south of the equator will continue to rise. Storms will follow the warming waters into the higher latitudes, dragging behind them the dry air that currently settles over the world's deserts.

Big risks

The models predict prolonged drought conditions in western US, with rainfall reducing by about 3.6 centimetres each year until 2150. "We always think of drought as being an occasional thing, but it's not going to be like that in the future," says team member Ming Fang Ting. "It's going to be dry all the time."

If the models are accurate, the western US will not be the only region affected. Semi-arid areas like those in eastern Australia, southern Africa, and the Mediterranean are all at risk.

Plants are partly to blame. Vegetation is crucial to sustaining atmospheric moisture in dry climates, says Gemma Narisma at the University of Wisconsin, Madison, US, who was not involved in the study. Plants react to reduction in available water by decreasing their moisture exchange with the atmosphere, which in turn leads to drier conditions.

To better understand this "positive feedback loop", Narisma recently studied thirty or so regions around the world that endured droughts during the 20th century that lasted for more than five years (Geophysical Research Letters vol 34, p L06710).

Recent trend

Narisma thinks the plant effect could help trigger the onset of the dustbowl-like conditions predicted by climate models, while the ensuing decades-long droughts would be sustained by changes to the atmospheric circulation.

Whether or not this predicted effect can be linked to the last seven years of dry climate in the western US is still up for debate. "Since the late 1990s, precipitation has trended downward in much of the western US," says Aiguo Dai of the National Center for Atmospheric Research in Colorado, US. "But over the last 50 years, much of the US, including the west has [seen] increased precipitation."

Isaac Held of the National Oceanic and Atmospheric Administration also urges caution in interpreting the models' dire predictions. "There is a hint that the current drying may be caused by what the models suggest, but it's only a hint right now," he says.

Journal reference: Science (DOI: 10.1126/science.1139601)

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